In the wireless communication industry, a base station is typically connected to a transmission tower using 50 ohm coaxial cable. Transmission towers are frequently the target of lightning strikes. Despite best efforts to adequately ground the towers, occasionally high voltage surges are transmitted through the coaxial cable. If the high voltage surge is permitted to be picked up by the center conductor of the coaxial cable and transmitted along the distribution network, electrical devices within the interconnects and along the distribution path would become inoperable due to the electrical components essentially melting or otherwise deteriorating as a consequence of the surge. Replacing the components can be expensive, time-consuming, and result in down-time for the cellular tower operator. To mitigate the effect of lightning strikes on the antenna tower, a surge protector is typically installed in line with the coaxial cable to prevent the passage of dangerous surges and spikes that could damage electronic equipment. During normal operation, microwave and radio frequency signals are passed through the surge protector without interruption. In the event of a lightning strike or other surge in voltage and/or current, the surge protector shunts the surge to ground.
One type of surge protector used in the coaxial cable for antenna towers is a quarter wave stub device, which has a tee-shaped configuration including a coaxial through-section and a quarter-wave stub connected perpendicular to a middle portion of the coaxial through-section. The coaxial through-section is mated at either end with a standard connector. At the tee-shaped junction between the stub and the coaxial through-section, the center conductor and outer conductor of the stub are connected to the center and outer conductors of the coaxial through-section. At the terminal end of the stub, the center and outer conductors are connected together, thereby creating a short, which is connected to ground. The physical length of the stub is equal to one-quarter of the center frequency wavelength for the band of frequencies passing through the coaxial cable.
During normal operation, the quarter wave stub device permits signals within the desired frequency band to pass through the through-section. A portion of the desired signal encounters the stub portion at the tee junction and is scattered down the length of the stub, where it is reflected off the short-circuit and travels back to tee junction. Because the physical length of the stub is equal to one-quarter of the center frequency wavelength for the band of frequencies passing through the coaxial cable, the scattered signal portion adds in phase to the non-scattered signal portion and passes through to the opposite end of the coaxial through-section.
When a surge occurs in the transmission line, such as from a lightning strike, the physical length of the stub is much shorter than one-quarter of the center frequency wavelength because the surge is at a much lower frequency than the desired band of operating frequencies. Thus, the surge travels along the inner conductor of the coaxial through-section to the stub, through the stub to the short-circuit, and through the short-circuit to ground. Thus, the surge is diverted to ground by the surge protector.
One drawback to the quarter wave stub device is that it has limited capability to pass dc signals. This is a problem for cellular transmission towers that have tower-mounted amplifiers, where it may be necessary to pass up to 90 volts from the base station up to the tower through the coaxial cable.
Another drawback the to the quarter wave stub is that it has a limited operating bandwidth, passing only a narrow band of frequency signals. With the growing resistance from communities to add more cellular towers, many cellular carriers are co-locating their operating systems by duplexing or even triplexing their respective frequency bands. In this manner, the different frequency spectrum for each carrier are combined at the top of the tower, sent through a common broadband coaxial cable to the bottom of the tower, and split off to their respective antennas and radios. If a quarter wave stub is installed in the broadband coaxial line, it will pass only a small a small range of frequency signals and filter out the rest, thereby acting as a narrow pass band filter. This is completely undesirable if a particular carrier's signals are within the filtered range.
Co-located carriers may also run their own individual coaxial cable from the tower to the base station, but this approach is wasteful and requires wireless service providers or tower operators to stock a range of quarter wave stub surge protectors to accommodate all the commonly allocated operating bandwidths (e.g., 800-870 MHz, 824-896 MHz, 870-960 MHz, 1425-1535 MHz, 1700-1900 MHz, 1850-1990 MHz, 2110-2170 MHz, 2300-2485 MHz, etc.).
Another type of surge protector installed in-line with coaxial cable for antenna towers is the gas tube arrestor. A gas tube arrestor typically contains a gas capsule placed in between the center conductor and the outer conductor in the coaxial line. The gas in the tube is normally inert, but ionizes and becomes conductive when a threshold voltage potential is applied across it. The gas tube arrestor allows the operating signals to pass through the device under normal operation but, in the event of a surge, the gas ionizes and creates a current path from the center conductor to the outer conductor, thus shunting the surge to ground. When the voltage potential across the tube decreases below the threshold, the gas in the tube becomes inert again.
One drawback with gas tube arrestors is that the response time of the device allows a voltage spike to pass through the device in the time period before the gas ionizes and becomes conductive. Although this time period is only milliseconds, voltages as high as 1 kV may be passed through to equipment at the base station, which may be detrimental to the equipment.
Another drawback to gas tube arrestors is that, over time and with multiple surge events, the gas in the tube remains somewhat conductive and may “leak” current to ground. Also, there is no way of determining if the condition of the device is deteriorated until it fails to work during a surge event. Therefore, manufacturers recommend periodic replacement of the gas tube arrestors regardless of their condition, which wastes time, manpower, and money.